1. Field of the Invention
The present invention relates to spectroscopy, and more particularly to a spectroscopy analyzer using a detector array without requiring focusing optics, and having no optical path exposed to the atmosphere.
2. Description of the Prior Art
Several structures for multiple internal reflectance crystals are known in the art. For example, radiant energy, such as infrared (IR) energy, may enter a crystal through a first beveled face so as to reflect off a first side. The energy reflects between the first side and a second side down the length of the crystal by the physical phenomenon of total internal reflection. A sample that is placed against either the first or second side of the crystal selectively absorbs different frequencies of energy. The energy that is not absorbed exits the crystal through a second beveled face to a detector that measures the distribution of energy absorbed by the sample so as to obtain its spectrum.
Attenuated total reflection (ATR) is a technique of analyzing a sample material using infrared reflection. ATR allows an infrared measurement to be made in the midinfrared region over a very short optical path. The depth to which incident energy penetrates a sample depends on the refractive index of the sample and the multiple reflectance crystal, as well as the angle of incidence at which the energy reflects off of the side of the crystal that is in contact with the sample. Changing the angle at which energy enters the crystal, i.e., the entrance angle, may change the angle of incidence. A multiple internal reflectance crystal, however, introduces chromatic aberration into the resulting distribution of energy if the incident energy is not normal to the surface of an entrance face of the crystal.
U.S. Pat. No. 4,730,882 to Messerschmidt, entitled xe2x80x9cMultiple Internal Reflectance Spectroscopy Systemxe2x80x9d, describes a design where any one of several multiple internal reflectance crystals, having different angles of incidence, can be positioned at a location that is remote from a source and receiver of radiant energy without the need for a realignment of transfer optics. A multiple internal reflectance crystal in accordance with the Messerschmidt patent has a sample surface and a bottom surface and reflective beveled ends such that energy may enter normal to the bottom surface, reflect off one beveled end to the bottom surface, from the bottom surface to the top surface, down the length of the crystal, and exit the crystal normal to the bottom surface by reflecting off of another beveled end.
A typical infrared spectrometer consists of a source of infrared radiation, a sample chamber where an exchange of energy takes place between the radiation and the sample, a means of dispersing the infrared radiation, i.e., a dispersing means such as a prism, a grating or an interferometer, and a detector that measures the energy level from the dispersing means. When the dispersing means is a prism or a grating, the dispersed radiation is scanned across a slit from whence it is focused on a detector. The spectrometer also includes focusing optics, such as a series of mirrors, some of which are aspheric, to focus light energy from the source onto the entrance face of the sample chamber, and from the sample chamber to the dispersing means and from thence through the exit slit to the detector. The total optical path may be a meter or more. Because of the long optical path, the spectrometer must be purged with a nitrogen gas or evacuated to eliminate absorption from atmospheric gasses such as CO2 and water.
Dispersion shifts occur with temperature change for all three dispersion methods, i.e., prism, grating and interferometer. Accordingly, the spectrometer must be maintained at a constant temperature.
A radiation detector can be a detector array, which consists of a number of detector elements located adjacent to each other. A grating focuses a slit image in the form of a dispersed band to illuminate the array with dispersed infrared radiation. However, such a design may involve several aspheric mirrors and an optical path of significant length. Temperature changes may also cause the dispersed band to move across the array, thus changing the wavelength distribution on the detector elements.
There is a need for a spectrometer that does not require focusing optics.
There is also a need for such a spectrometer that minimizes the length of the optical path.
There is also a need for such a spectrometer, the operation of which is not susceptible to a temperature variation.
In accordance with the present invention, an apparatus is provided for analyzing a spectrum. The apparatus includes an elongated source of light, a device for producing a spectrum of the light, a sample stage, and an array of photosensitive elements for detecting the spectrum and providing an output representative of an intensity of the spectrum as a function of wavelength. The sample stage is interposed between the elongated source and the spectrum-producing device. The light propagates along a length of the sample stage from the elongated source to the spectrum-producing device and thereafter to the array. The elongated source has a length greater than or equal to a length of the ar